Use of Dielectric Spectroscopy to Estimate the Condition of Cellulose-Based Insulation L.V. Badicu * , P.V. Notingher * , L.M. Dumitran * , G. Tanasescu ** , D. Popa *** * University Politehnica of Bucharest, 313 Splaiul Independentei St., 060042, Bucharest, Romania, E-Mail: lvbadicu@elmat.pub.ro ** SIMTECH INTERNATIONAL Ltd, 111 Constantin Brancoveanu Blvd., Bucharest 4, Romania E-Mail: gabriel_tanasescu@yahoo.com *** ICMET Craiova, 144 Calea Bucuresti, 200515 Craiova, Romania, E-Mail: dpopa@icmet.ro Abstract - Assessment of the power transformers insulation condition is a permanent concern for carriers and distributors of electrical energy. Different methods for their monitoring and diagnosis were developed in recent years. The aim of this work is to present the basis of the dielectric spectroscopy method and the first results obtained in time-domain and in frequency domain that permit to estimate the moisture content in pressboard. At the end, the possibility to use this method for off-line and on-line transformer monitoring is analyzed. Keywords: dielectric spectroscopy, cellulosic insulation, power transformers, absorption and resorption currents. I. INTRODUCTION Cellulosic materials form the basis elements of power transformers insulation systems. If in 1939 the cellulose insulation were used especially for paper capacitors, paper-insulated power cables, telephone cables and the annual consumption was estimated at 18 thousand tons, nowadays the annual consumption is measured in million tons, especially for oil filled HV power cables, condenser bushings and power transformers. Cellulosic insulation is used primarily in oil-filled transformers from distribution to large power units covering a wide range from 10 kVA to 1500 MVA and from line voltage to 1000 kV. In terms of physical size, it ranges from the pole and pad – mounted units on our streets to large substation units that can have several tons of cellulosic insulation (paper and pressboard) immersed in 40.000 to 100.000 l of oil. The insulation structure consists of not only the HV and LV insulation but also different prefabricated items: support structures, winding tubes, spacer blocks, and formed items for end closing [1]. Electrical grade paper and pressboard are mostly made from wood pulp processed by the Kraft board. Cellulose, the essential component of paper and pressboard, is a polymer of glucose units linked to one another in a special manner as shown in Fig. 1. It may be represented simply as [C 5 H 10 O 5 ] n , ignoring the extra atoms on the end groups, where n is the degree of polymerization (DP). The DP values for paper samples of Kraft pulps range from 1100 to 1200 but mixed pulp fibers can have much higher DP (1400…1600) [1]. Fig. 1. Chemical structure of cellulose polymer. The most important disadvantage of cellulosic materials for electrical use is that they are hygroscopic and need to be processed and maintained dry. At room temperature (20 – 25 ºC) cellulose can hold from 4 to 8 % moisture in the relative humidity range of 30 to 70 % typical on factory floors in winter and summer conditions. The moisture level in insulation in a newly built transformer should be about 0.5 %. In a wet insulation partial discharges inception becomes significant above 3 % moisture level (and may result in gas bubbles and release of hydrogen), insulation power factor would be above acceptance limits, paper degradation an aging be excessive. For regular Kraft paper the life would be lowered by half for every doubling of moisture content, though for upgraded paper the loss of life is not so drastic. From a temperature perspective, the temperature rise above 75 ºC for the mechanical strength to decrease to half is dependent on the moisture content of the paper (for 1 % moisture content, the doubling interval is for every 24 ºC, and for 8 % moisture it is 8 ºC). Degradation products based on cellulose is the exhibition of three chemical important reactions: pyrolisis (resulting small chain, CO, CO 2 , H 2 O, and furans), oxidation (resulting CO, CO 2 , H 2 O and acids) and hydrolyze (resulting smaller chains). Ageing of cellulose would lower the DP by depolimerization, which involves breakage of the linkage by hydrolytic 7